Limit switch



Nov. 20, 1945. D. w. EXNER 2,389,350

LIMIT SWITCH Filed Aug. 6, 1942 WITNESSES: INVENTOR E flo/m/d .Exner.

ATTORNEY atentefi New 3%5 LIIVHT SWITCH Application August 6, 1942, Serial No. 453,773

3 Claims. (01. 200-47) My invention relates, generally, to travel-limit switches and, more particularly, to limit switches of the continuous drive type.

An object of my invention is to provide a limit switch which will permit a long travel of the controlled device but still maintain accurate control of the stopping points.

When the limit switch is engaged with the motion of the main device only as the motion approaches its travel limit, accurate control of the point of stopping is an easy matter. An example of this type is the well known track limit switch in which the switch is tripped when a projection on a car or table engages the operating arm of the limit switch, thereby stopping the moving member.

Frequently the foregoing method cannot be adapted to a particular mechanism, as, for example, a hoist, where it is impractical to attach anything to the hoist cable for actuating a limit switch. In such a case it is more convenient to use a self-contained limit switch which is driven continuously by the hoist drum. While continuous-drive limit switches of the traveling-nut type are well known, to obtain accurate control of the stopping points for long travel requires a long lead-screw or worm. Certain refinements in switches of this type have reduced the length of lead-screw required, but the space occupied is still objectionably large.

Accordingly, a further object of my invention is to provide a limit switch of the continuousdrive type which shall be simple and eflicient in operation and which requires a relatively small amount of space.

Another object of my invention is to provide a limit switch suitable for controlling movement in either one or both directions. a

A still further object of my invention is to provide a limit switch in which the distance of travel may be readily adjusted.

Other objects of my invention will be explained fully hereinafter or will be apparent to those skilled in the art.

In accordance with my invention, two cams are driven at different speeds by suitable gearing. The slower cam makes not more than one revolution over the entire range of motion to be limited. The second cam rotates considerably faster than the first, for example times faster.

An electrical switch is provided with a rider shoe which engages the peripheries of both cams. Each cam has a notch into which the shoe may fall, thus operating the switch when the notches in the two cams coincide under the shoe. At other points in the travel the switch is held inoperative because the shoe rides on the outer periphery of either one or both cams. Thus, the slow cam selects the general portion of the travel in which the switch is to operate, while the fast cam selects the exact point of operation.

For a fuller understanding of the nature and objects of my invention, reference may be had to the following detailed description, taken in conjunction with the accompanying drawing, in which:

Figure 1 is a view, in elevation, of a limit switch embodying my invention, portions of the structure being removed for clearness; and.

Fig. 2 is a view, in section, taken along the line IIII of Fig. 1.

Referring to the drawing, the structure shown therein comprises a box ill for housing a plurality of coaxial cams H, l2, l3 and H. The box III is provided with a removable cover IS. The cams l2 and I3 are mounted on a quill l6 which is driven at a relatively slow speed by intermeshing gears l1 and I8. The cams H and I4 are mounted on a shaft H which is disposed inside the quill l6 and driven at a considerably faster speed by intermeshing gears 2| and 22. By way of example, the gear ratios may be such that the cams H and 14 rotate ten times as fast as the cams l2 and l3.

The gears l8 and 2| are mounted on a shaft 23 driven by a worm wheel 24 that meshes with a worm gear 24a driven by a shaft 25. The shaft 25 may be connected to the drum of a hoist or ing 29 is disposed in the bottom wall of the box ID. The quill I6 is rotatably mounted in a bear ing 33 disposed in the plate 3|. The quill functions as a bearing for the shaft I 9, one end of which is mounted in a bearing 34 disposed in the bottom wall of the box I 0. The cam II is carried by a circular bracket 35 which is secured to the other end of the shaft l9 by a nut 36. The cam I2 is secured to the quill It by a nut 31 and the cam l3 and the gear I! are secured to the quill It by screws 38. The cam l4 may be pressed on a sleeve of the gear 22 which, in turn, may

be pressed on the shaft l9. Thus, the cams II and I4 rotate with the shaft l9 while the cams l2 and I3 rotate with the quill Hi.

As explained hereinbefore, the shaft l9 may be geared to rotate ten times as fast as the quill l6. Thus, one of the pair of adjacent cams H and 12 makes ten revolutions while the other is making one revolution. Likewise, one of the pair of adjacent cams l3 and I4 revolves ten times while the other is revolving once.

As shown in Fig. 1, an electrical switch assembly 42 is disposed to be operated by the cams II and I2 and a similar switch assembly 41 is disposed for operation by the earns 43 and I4. The switch assembly 42 comprises a pivotally mounted contact arm 43 for engaging either a fixed contact member 44 or a fixed contact member 45. A compression spring 46 engages the arm 43 to bias the contact arm'43 towards the contact member 45. A rider shoe 41 is so attached to the arm 43 that it rides on the peripheries of both cams H and I2 because of the pressure of the spring 46. Each cam has a notch 48 into which the shoe may fall, thereby operating the switch to close the contact members 44 and 45, when the notches in the two cams coincide. At all other times during the rotation of the cams contact members are held open because the shoe rides on the outer periphery of either one or both cams.

Thus, the slow cam selects the general portion of the travel in which the switch is to operate, while the fast cam selects the exact point of operation. It will be understood that suitable electrical circuits may be connected to the contact members of the switch assembly for controlling the operation of the controlled device, such as a motor-driven hoist. Terminal connectors and 52 are provided for making the electrical connections. The necessary connections may be brought into the box It! through a plug 49 in one side of the box.

The switch assembly 4| is similar to the assembly 42 and comprises a pivotally mounted contact arm 53 for engaging either a fixed contact member 54 or a fixed contact member 55. A compression spring 56 engages the arm 53 to bias the contact arm 53 towards the contact member 55. A rider shoe 51 engages the peripheries of both cams l3 and i4. Each of these cams has a notch, similar to the notch 48 in the cams I l and I2, into which the shoe 5'! may fall, thereby closing the contact members 54 and 55 when the notches in the two cams coincide.

Thus, by providing a separate pair of fast and slow cams to operate a switch to limit each end of travel of the controlled device its operation in both directions may be controlled. For example, both the raising and the lowering operations of a hoist may be controlled by a limit switch of the type herein described. By making the angular position of each cam on its shaft adjustable, the operating point of each switch may be readily adjusted to suit the device to which the limit switch is applied.

Figures 1 and 2 illustrate an arrangement of cams on a limit switch which requires an adjustable limit in one direction only. A typical application for such a device is on a hoist in which only the upper limit need be adjusted to compensate for changes in cable length, while the lower limit may be permanently set at the time of assembly to prevent tearing the end of the cable from its attachment to the drum. In the case shown, only cams II and 12 are readily adjustable in position; cams l3 and I4 being adjustable less easily by changing the meshing of the gearing. By suitable modification of the structure, both pairs of cams can be made readily adjustable, when that is considered desirable.

It will be understood that the eilective length of base-line, or cam peripheral distance for each pair of cams equals the usable portion of the circumference of the slow cam multiplied by the ratio of the speed of the fast cam to the speed of the slow cam. Thus, if this ratio is 10:1 the use of my invention provides results equivalent to the use of a single cam having a diameter approximately ten times as great.

From the foregoing description it is apparent that I have provided a switch which may be utilized for controlling the limit of travel of devices of various types. The present switch is relatively simple in structure and small in size. Therefore, it may be utilized without materially increasing the cost and size of the controlled device.

Since many modifications may be made in the apparatus and arrangement of parts without departing from the spirit of my invention, I do not wish to be limited other than by the scope of the appended claims.

I claim as my invention:

1. In a device of the class described, in combination, a plurality of pairs of adjacent cams, means driven by the same source for continuously rotating the cams of each pair at different speeds first in one direction and then in the other direction, switching means actuated by each pair of adjacent cams independently of the other pair when one cam has made a predetermined number of revolutions in the same direction as and relative to the other cam of said pair, and means for adjusting the relative angular positions of the cams of each pair.

2. In a limit switch, in combination, a plurality of pairs of adjacent cams, means driven by the same source for continuously rotating the cams of each pair at different speeds first in one direction and then in the other direction, a switching member for. each pair of adjacent cams, said switching members being actuated independently as a result of one cam of each pair making a predetermined number of revolutions in the same direction as and relative to the other cam of said pair, and means for adjusting the relative angular positions of the cams of each pair.

3. In a limit switch, in combination, a plurality of pairs of coaxial cams, means driven by the same source for continuously rotating the cams of each pair at difierent speeds first in one direction and then in the other direction, a springbiased follower for each pair of cams, switching means actuated independently by said followers as a result of one cam of each pair making a predetermined number of revolutions in the same direction as and relative to the other cam of said pair, and means for adjusting the relative angular positions of the cams of each pair.

DONALD W. EXNER. 

